Assuming that you have other cattle, you should probably vaccinate any new ones as soon as they arrive, or the first chance that you get. If you vaccinate them upon arrival, you can always give them a booster with your usual annual or semi-annual roundup. If you have no other cattle, then waiting until the fall is reasonable. The plan that TXCOWBOY laid out is pretty good. Just make sure that you at least hit them with vibrio., lepto., blackleg (clostridium), and deworm them.